Vibration damping devices such as vibration damping linkages or vibration damping supports have been used to be interposed between components that make up a vibration transmission system. One type of known device is a vibration damping device having a first mounting member and a second mounting member linked by a main rubber elastic body, and a fluid filled type vibration damping device is a developed type thereof. This fluid filled type vibration damping device includes a pressure receiving chamber whose wall is partially defined by a main rubber elastic body; and an equilibrium chamber whose wall is partially defined by a flexible film, the two chambers being filled with a non-compressible fluid, and the two chambers communicating with each other through an orifice passage. This construction can exhibit vibration damping effect during vibration input to the pressure receiving chamber due to resonance or other flow action of the fluid induced to flow through the orifice passage on the basis of pressure differential between the pressure receiving chamber and the equilibrium chamber. The application of such fluid filled type vibration damping devices in automotive engine mounts, body mounts, differential mounts, suspension mounts, and suspension bushings for example, is a topic of ongoing research.
In an application such as an automotive engine mount, respective vibration damping effects against vibrations in multiple frequency ranges are required. To meet this requirement, in general, there have been proposed a construction in which an orifice passage is tuned to low-frequency, large-amplitude vibration such as engine shake while a movable film is provided for absorbing pressure fluctuations in a pressure receiving chamber to cope with high-frequency, small-amplitude vibration such as driving rumble.
Additionally, in recent years, an application such as an automotive engine mount has a problem of occurrence of vibration or noise during input of excessive vibration load or jarring load. This is thought to be caused mainly by cavitation bubbles in association with excessive negative pressure arising in the pressure receiving chamber. Specifically, when large-amplitude vibration is input to induce a condition of excessive negative pressure in the pressure receiving chamber, the air dissolved in the fluid of the pressure receiving chamber undergoes liquid phase separation to form cavitation bubbles. It is conceivable that water hammer pressure accompanying bursts of the bubbles will be propagated through the first mounting member and the second mounting member and transmitted to components such as the car body that make up a vibration transmission system, producing noise or vibration which can be a problem.
In order to address this problem, in Patent Citation 1 (Japanese Patent Application No. 2007-311749), the Applicant proposed a novel structure in which a partition member that partitions a pressure receiving chamber and an equilibrium chamber is provided with a communication passage connecting the two chambers, and an obstructing rubber elastic plate is superposed against the communication passage from a pressure receiving chamber side so as to obstruct the communication passage, thereby defining controlling means for opening/closing the communication passage. With this structure, when a sudden pressure drop occurs in the pressure receiving chamber during input of excessive vibration load or jarring load, the obstructing rubber elastic plate undergoes elastic deformation to be spaced away from the partition member to place the communication passage in the open state. The pressure receiving chamber and the equilibrium chamber are then short circuited, whereby negative pressure can be prevented from arising in the pressure receiving chamber. Moreover, the obstructing rubber elastic plate can exhibit vibration damping effect against high-frequency, small-amplitude vibration by absorbing pressure fluctuations through elastic deformation with the communication passage closed.
Research conducted by the inventors as to the fluid filled type vibration damping device disclosed in Patent Citation 1 led to the idea that there is clearly room for further improvement. Specifically, with the opening/closing control means disclosed in Patent Citation 1, during input of vibration such as engine shake, the obstructing rubber elastic plate that covers the communication passage undergoes elastic deformation on the basis of pressure differential between front and back sides thereof and pressure in the pressure receiving chamber will be absorbed in association with the elastic deformation. There is a consequent risk that the amount of fluid flow through the orifice passage will decrease, making it difficult to achieve sufficient vibration damping effect against low-frequency, large-amplitude vibration exhibited by the orifice passage.
To cope with this problem, it would be conceivable to endow the obstructing rubber elastic plate with greater elastic rigidity. However, this arrangement poses a problem that pressure fluctuations in the pressure receiving chamber may not be reduced during input of high-frequency, small-amplitude vibration such as driving rumble, causing deterioration of vibration damping ability against high-frequency, small-amplitude vibration.
That is, in some instances, the fluid filled type vibration damping device disclosed in Patent Citation 1 cannot be sufficient to meet the requirements of achieving both of (i) vibration damping effect against low-frequency, large-amplitude vibration exhibited by the orifice passage and (ii) vibration damping effect against high-frequency, small-amplitude vibration exhibited by elastic deformation of the obstructing rubber elastic plate, and further achieving (iii) inhibition of impact or noise caused by pressure fluctuations in the pressure receiving chamber in association with input of excessive vibration.